Communications and radar systems have long employed the parabolic antenna for transmission and reception of high frequency RF high frequency electromagnetic energy in the microwave and higher frequency ranges. That antenna at a minimum contains two principal elements: An RF feed, through which the antenna is electromagnetically coupled to associated transmitting and/or receiving apparatus; and a reflector, a surface of parabolic shape, formed of a material that reflects RF, spaced from that feed. More complicated antennas are known that contain additional elements, including additional reflectors.
Since RF energy in the microwave spectrum and higher frequencies propagates, like light, in a straight line, the parabolic surface reflects RF that travels coaxially with the reflector's axis and is incident anywhere on the reflector's parabolic surface, converging that RF to the parabola's focal point, where the RF feed is positioned. Thus, RF energy that may travel in separate parallel paths to the reflector is concentrated at the feed, producing a stronger more intense RF signal.
More modern antennas of that type, referred to as an off-set parabolic antenna, differ slightly from that structure. Instead of employing an entire paraboloid as a reflector, only an offset section of the paraboloid is used. That section of paraboloid may be visualized as the intersection of a right cylinder extending axially, but off-set from the parabolic axis, and the paraboloid's surface. The intersection of the cylinder and the paraboloid forms an ellipse lying on a plane. That ellipse appears circular in outline as viewed from the axis of the imaginary right cylinder. That section of the paraboloid physically resembles a small concave shaped saucer, hence the given reference as a "dish".
Retaining the reflective characteristic of the parabolic surface, the dish reflects incident RF energy propagating parallel to the parabolic axis from any location on the surface to the RF feed at the focal point, the latter of which is physically off-set from the dish. Because the feed is offset, there is no blockage of the reflective surface, as could induce side lobes in the RF signal. Since extraneous RF signals could be introduced to the antenna system through side lobes and create electronic noise in the associated receiving apparatus, minimization or elimination of side lobes is desirable.
A principal application for parabolic antennas of either type is in conjunction with communications and/or radar systems on board spacecraft, where weight and storage space are at a premium. Accordingly, antennas for spacecraft application must be as light in weight as technology and materials science permits, which minimizes the direct and indirect propellant fuel requirements and costs to launch and carry the antenna into outer space.
The antennas must also be strong enough to meet structural design requirements, particularly as to stiffness and strength. They must also collapse or, as variously termed, fold up for storage and then, essentially, on command, unfold to a substantially larger size when deployed. The capability to fold up minimizes the volume of space occupied by the antenna in the spacecraft during its transport, a structural characteristic that is referred to as deployable. It should be understood that when an element is referred to herein as deployable, the intended meaning is that the element folds up into a smaller size, its undeployed or stowed size, and unfolds to a larger size, its deployed size.
To achieve deployability, collapsible or foldable reflectors, as variously termed, were developed and applied in the past to spacecraft as a component of the spacecraft's antenna system. Once such prior mechanism is an umbrella-like reflector structure, which, like a household umbrella, unfolds radially outwardly extending spokes of curved geometry that support a pliant reflective surface, typically a metal mesh, that stretches into the required curved shape.
Another is a perimeter truss reflector, such as is found in U.S. Pat. No. 5,680,145 granted Oct. 21, 1997 to Thomson et al, assigned to Astro Aerospace Corp, hereafter sometimes referred to as the "Thomson" reflector, to which patent the reader may refer for additional background. The principal elements of the Thompson reflector are the perimeter truss, the reflective material and the geodesic structure, including a shaping system, that supports the reflective material and shapes the reflective material into a concave parabolic shape. The reflectors described herein are also of the latter type.
As deployed, in appearance, the perimeter truss forms a large diameter short hollow cylinder. Its cylindrical wall is pervious and comprises a skeletal frame of tubular members in a closed loop, that in many respects is reminiscent of the frame of a steel skyscraper, but with the top end of the skyscraper's frame wrapped around into a circle and joined to its bottom end.
The reflective surface supported on the truss is either a pliant metal gauze, mesh, cloth-like material or a thin metalized membrane, or of any other form as well, all of which may collectively be referred to as pliant reflective material. Where a mesh material is selected, at the higher RF frequencies the mesh material is formed of very fine gold plated filaments joined in a fine mesh that resembles women's nylon stockings and is almost invisible to the eye. At the lower RF frequencies the mesh is more coarse in nature and resembles chicken coop wire in appearance. Such pliant reflective material is well known in the deployable antenna art.
To mold and shape as well as to hold the reflective surface in place on the truss, typically, the front and rear ends of the truss contains a geodesic backup structure or a series of tension lines, termed catenaries, that structurally define the parabolic surface in a skeletal or wire form. The catenaries extend across the end of the truss and are supported at the trusses end edges.
The catenaries located on the trusses front end overlie and are aligned with like catenaries supported on the trusses rear end. By tying or otherwise connecting various points along a single catenary to like points on the underlying catenary with ties that judiciously differ in length, each catenary may be shaped to approximate a portion of a parabolic curve. By judiciously shaping each catenary in the series to an appropriate portion of a parabolic curve, a entire parabolic surface is skeletally defined. That skeletal surface serves as the wall, seat or bed, however characterized, on which the reflective surface is placed, somewhat like a bed sheet laid upon a bed or a tissue blown against a window screen.
The reflective material contains some means to permit attachment or coupling to an underlying catenary. Suitably that material is attached or coupled to downwardly extending pliant drop lines or ties, which tie the reflective material to the underlying catenary member. Thus, the pliant material in these perimeter truss antennas is stretched taut to achieve the desired concave shape with an acceptable smoothness in surface defined by the shaping system when the deployable rigid frame members supporting the shaping system are extended to their deployed position. Like one's umbrella, the reflective material should drape and be collected together by moving the deployable rigid frame members to a stowed position.
For spacecraft operation, the perimeter truss is also required to be sufficiently stiff so that, as deployed, any natural modal frequencies which might be excited in the reflector as a consequence of spacecraft maneuvering or other on-orbit disturbances, as might disrupt the spacecraft's mission, are quickly damped out. Also, low frequency oscillations of the truss could adversely affect the spacecraft's orientation control apparatus.
The prior truss reflector described in the cited '145 Thomson et al patent employs, on both the front and rear of the truss, tension members or lines, which are essentially pliant, are arranged in a geodesic or crisscrossing pattern, creating multiple facets, and that pattern is pre-configured into the desired concave shape. Each geodesic pattern is tensioned with soft metal springs that connect at each intersection of the crisscrossing tape or lines. The size and number of facets in that geodesic system is governed by the highest frequency of RF that the antenna is designed to handle. The higher the frequency, the greater the number of facets required, and, hence, the greater the number of such metal springs required.
The present invention recognizes that the foregoing produces a heavier antenna structure than desired. As an advantage, the new perimeter truss reflectors described herein provides a weight saving compared to the foregoing structure, for one, by eliminating the crisscrossing lengths of catenaries, and metal springs.
Further, when deployed, the Thomson reflector forms a flat circular band. Such a geometry is inherently unstable in the out-of-plane bending direction. In other words, the circular band possesses little resistance, should external forces try to bend or twist the band into a potato chip like shape. To achieve on-orbit frequency requirements, the Thomson truss is stabilized by the geodesic system that supports the reflective mesh.
In contrast, perimeter trusses described in this specification are inherently stable to such bending or twisting forces. Its frame is sufficiently stiff to meet on-orbit frequency requirements on its own and, unlike the Thomson reflector, does not depend on the reflective material's support system to achieve out-of-plane stiffness. An ancillary consequence of that new found independence and as a further advantage to the invention is that trusses made in accordance with the invention may use a simple light weight catenary system to support the reflective mesh material to the truss, thereby further reducing the reflector's weight.
The means by which the Thomson et al reflector folds-up to attain its stowed condition dictates its stowed height, that is, the height of the package when the reflector is in the non-deployed or stowed configuration. As realized, the greater the space taken to stow the reflector, the less space remains available on-board the spacecraft for other equipment, or, conversely, given the requirement for other on-board equipment and only a pre-alloted space available for the antenna, the size of the reflector that may be stored in that space is limited.
As an additional advantage, the present invention reduces stowed package size for a given size reflector in comparison to the prior designs. As becomes apparent from the description of the preferred embodiments, which follows in this specification, for a given size antenna, the present invention stows more compactly than a Thomson reflector of the same size.
Accordingly, an object of the invention is to provide a new folding perimeter truss structure suitable for deployment in outer space.
A further object of the invention is to provide a folding perimeter truss structure that, for a given diameter, is of lighter weight than perimeter truss structures previously known.
A still further object of the invention is to provide a perimeter truss structure that has a size expansion ratio, the change in size from the undeployed to the deployed configuration, that is greater than previously attainable from prior perimeter truss reflectors.
An additional object of the invention is to provide a folding perimeter truss whose stiffness characteristic and/or rigidity is independent of the reflective mesh material's support system, and does not rely upon the latter element to attain sufficient stiffness.
A still additional object of the invention is to provide a folding perimeter truss structure that i s useful for supporting traditional symmetric parabolic reflectors as well as for offset parabolic reflectors.